1. Field of the Invention
The present invention relates to a heating device and an image forming apparatus, in which a heater provided in a fixing device is switched ON/OFF in response to a zero-cross point of an AC voltage outputted from an AC power source.
2. Description of the Related Art
An image forming apparatus such as a printer, a copying machine, a facsimile machine, and a complex machine having functions of those has a fixing device which applies a fixing processing to a toner image formed on a sheet. The fixing device generally has a heating roller and a pressing roller, and a sheet passes through a nip portion between the rollers, so that a toner image is fixed on the sheet.
In such heating roller of a fixing device, there is provided a heater, and a power supply from the AC power source to the heater is controlled by a switching device. In other words, the power supply to the heater is switched ON/OFF by the switching device, so that the temperature of the heater is controlled. Further, it is known that a switching noise occurs when the switching device switches ON/OFF. To avoid an effect of the switching noise, the switching device is controlled to switch ON/OFF at a timing (zero-cross point) at which an output of the AC power source becomes zero.
However, when a switching control is performed with respect to such a great load of a heater of a heating roller, the output of the AC power source is changed by a noise. The change in the AC power source causes a zero-cross point to be detected falsely, so that the heater control has been interfered.
In view of such phenomenon, Japanese Patent No. 3636254 discloses a technology related to a fixing device using a phase control to control the temperature of a heater. According to this technology, a zero-cross interruption timing and a switching timing are controlled to be in one-to-one relationship, and a zero-cross interruption due to a noise is disregarded even if the zero-cross interruption falls between the zero-cross interruption timing and the switching timing. Further, Japanese Unexamined Patent Publication No. 2002-272089, Japanese Examined Patent Application Publication No. 5-87844, Japanese Examined Patent Application Publication No. 7-72849, and Japanese Unexamined Patent Publication No. 4-165954 disclose a technology of providing a zero-cross interruption disregard period, taking the zero-cross interruption as a trigger, to perform a control of disregarding a zero-cross interruption due to a noise.